1. Field of the Invention
This invention relates to a luminescent material for an organic EL device having excellent light-emitting properties.
2. Description of the Related Art
An organic electroluminescent device (organic EL device) is a light emitting device, containing a fluorescent material which emits light in response to the recombination of holes and electrons that are injected from anode and cathode. For example, C. W. Tang et al. reported an organic EL device using a double-layered structure (Applied Physics Letters, 51,913 (1987)). This organic EL device, which has tris (8-hydroxyquinolinol aluminum) in a light-emitting layer and a triphenyldiamine derivative in hole-transporting layer, greatly enhances the luminescence properties.
To further improve luminescence efficiency, a method of doping a fluorescent dye had been utilized. For example, an organic EL device with a coumarin dye as the doping material (Applied Physics Letters, 65,3610 (1989)) has been used to greatly improve the luminescence efficiency. For improving the recombination efficiency of the injected hole and electron, multi-layered devices have been introduced. As a hole-transporting material, triphenylamine derivatives such as 4, 4xe2x80x2, 4xe2x80x3-tris (3-methylphenylphenylamino)-triphenylamine and aromatic diamine derivatives such as N, Nxe2x80x2-diphenyl-N, Nxe2x80x2 bis (3-methylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2diamine are well known (see Japanese Patent Application Laid-Open Nos. 20771/1996, 40995/1996, 40997/1996, 53397/1996, and 87122/1996). As an electron-transporting material, triazole derivatives and the like are also known.
Examples of the blue light emissive material for organic EL device include condensed polycyclic aromatic compounds such as anthracene, pyrene, and perylene (J. Chem. Phys., 44, 2902 (1996)), tetraphenylbutadiene-based compounds, distyrylbenzene-based compounds, stilbene-based compounds and polyphenyl-based compounds. (unexamined published Japanese patent application JP-A-61-37890, JP-A-1-245087, JP-A-2-247227, JP-A-2-247278, JP-A-2-209988, JP-A-3-33184, JP-A-3-84089, JP-A-3-231970, JP-A-4-117485, JP-A-4-275268, JP-A-5-17765, JP-A-140145, JP-A-3-47890, JP-A-3-33183, JP-A-5-247459, and JP-A-9-157642). However, these compounds have problems in durability, color purity, and luminescence efficiency.
An object of the present invention is to provide a high performance material and to provide an organic EL device having blue luminescence. The present invention uses a specific N-substituted carbazole compound as a light emitting material to provide an organic EL device having high color purity and luminescence. The organic EL device comprises an anode, cathode and one or more organic thin film layers which contain, either singly or as a mixture, an N-substituted carbazole compound represented by the following formula (1) or (2) 
Wherein R1 represents a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aromatic hydrocarbon group, a substituted or unsubstituted aromatic heterocyclic group, a substituted or unsubstituted amino group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, or a substituted or unsubstituted alkoxycarbonyl group; and Rx is one or more functional groups represented by a hydrogen atom, halogen atom, nitro group, cyano group, carboxyl group, or R1. Any two Rx groups may form a ring. X represents an oxygen atom, an NH group or a sulfur atom. A blue organic EL device can be provided according to the present invention.